Simple automotive air conditioning systems have long been provided with a temperature responsive switch to control operation of the compressor clutch with the advantage that no energy is wasted in driving the compressor when compression is not required. Switches for controlling clutch operation are therefore quite old.
In recent times, automotive air conditioning systems have become rather sophisticated and have involved operation of the compressor at all times with some penalty by way of energy consumption. With the growing emphasis on fuel economy, there is renewed interest in the on/off type operation of the compressor. To satisfy a public accustomed to relatively close control of the temperature in the air conditioned automobile, the operating characteristics demanded for the thermostatic switch have been made more stringent. In addition the switch must be capable of operating in rather hostile environments so far as dust and moisture are concerned and hold a set point after repeated cycling between ambient temperatures of -40.degree. to +135.degree. C. These requirements, therefore, occasion a fresh look at the problem of designing the control.